Stainless Steel Self Drilling (Tek) Screws – what are they?
The ‘Self Drilling Screw for Metal’ or ‘Stainless Steel Tek Screw’ is a screw designed to self-drill into metal. There are several material types available and they each have their strengths and weaknesses.
Why does the grade matter?
Getting the material right for your application is critical. Some Stainless grades have excellent corrosion resistance (304 and 316) but are relatively soft and will not self-drill through much more than thin aluminium. Other grades like 410 are heat treated and have excellent hardness – but anywhere near salt-water (or any sort of coastal atmosphere) and you will see some tea-staining pretty quick!
Some Stainless grades such as Hardtec and Marutex offer the benefits of both (hardness and excellent corrosion resistance) – but are only available in Hex Washer Face Head style. Getting the right material grade and screw type for your application is critical.
Stainless Steel Self Drilling (Tek) Screws – so what is available?
The Stainless Steel Self Drilling Screws are generally available in 4 main material types within the Stainless range, they are:
Bi-Metal Self Drilling Screws – these are not completely Stainless, the Bi-Metal screw generally has a 304 Stainless Steel body with a Carbon Steel tip. Because of the Carbon Steel tip they are very hard and good for drilling but they are not 100% Stainless Steel. They usually have a 500hrs Salt Spray Protective Ruspert Coating on them to increase corrosion resistance somewhat.
410 Stainless Self Drilling Screws – These are completely Stainless. 410 is a Martensitic Stainless and so can be hardened by heat treatment (which Austenitic types 304 and 316 cannot be). For this reason it has excellent drilling properties – however 410 has lower corrosion resistance than grade 304 and should not be used in highly or even moderately corrosive environments or tea-staining will result.
304 and also 316 Stainless Self Drilling Screws – Because the 300 series of Stainless Steels are of an Austenitic crystal structure – hardening by heat treatment like the 410 grade is not possible. These 300 series screws provide excellent corrosion resistance but are really only good for self-drilling through thin aluminium due to the softness of the grade. Also known as A2 (304) and A4 (316) in Europe.
Hardtec/Martuex Stainless – Both ‘Hardtec’ and ‘Marutex’ are propriety grades of Stainless Steel, made specifically for Stainless Self Drilling Screws. They are hardened by heat treatment and so have excellent drilling performance into metals like grade 410 BUT unlike grade 410, these grades have Molybdenum added to them and have corrosion resistant properties in line with the 300 series Stainless grades. Salt Spray tests show corrosion resistance equal or better to that of 304 grade and not far behind 316 Stainless Steel. They are an excellent screw!
Carbon Steel Tek Screws (Self Drilling Screws) are also available in Class 3 and Class 4 (with a galvanised surface) in Australia and New Zealand. These are commonly used in steel roofing. Aluminium versions are sometimes used for Aluminium cladding (to prevent Galvanic Corrosion from dissimilar metals). However Stainless Self-drilling screws provide the greatest corrosion resistance, particularly when the superior Hardtec grade is used.